All header fields which contain e-mail addresses only. Not all address fields have the same possibilities, but they are all parsed the same: you never know how broken the applications are which produce those messages.

When you try to create constructs which are not allowed for a certain kind of field, you will be warned.

The field is created with an utf8 string which only contains data from the specified character set. However, that character set can never be a valid name because it contains characters which are not permitted.

A new field is being created which does contain characters not permitted by the RFCs. Using this field in messages may break other e-mail clients or transfer agents, and therefore mutulate or extinguish your message.

Fatal error: the specific package (or one of its superclasses) does not implement this method where it should. This message means that some other related classes do implement this method however the class at hand does not. Probably you should investigate this and probably inform the author of the package.